1. Field of the Invention
The present invention relates to a field emission cold cathode device and more particularly to a device in which an electron beam sent forth from a field emission cold cathode element converges on a target.
2. Description of the Related Art
A field emission cold cathode element is an element comprising an emitter that is formed, on a substrate, in the shape of a pointed cone and a gate electrode that is provided with an opening of the order of submicron in the vicinity of this emitter, wherein a voltage applied, in a vacuum, between these two generates a high electric field centered on the apex of the emitter, and thereby electrons are emitted from the apex of the emitter.
Such a field emission cold cathode element is capable to produce a higher current density than a thermionic cathode element and therefore, taking advantage of this, the application thereof to devices in which an electron beam sent forth from this cold cathode element is required to converge on a target, for example, an electron-beam exposure system, a Braun tube, an electron-beam welding device, an electron-beam heating device and an electron-beam machining device is under consideration.
In such a device, electrons emitted from the emitter pass through an electron lens placed above the emitter with respect to the direction of the electron emission and converge on a target. FIG. 1 shows the basic structure thereof (Note that all the drawings shown herein below lie sideways). Electrons emitted from an emitter 1 travel in a vacuum in directions within a certain angle of divergence (an angle made between the travelling direction of the outermost electron when the kinetic energy of an electron is e.multidot.V and the line normal to the gate face is, hereinafter, referred to as the "electron beam divergence angle (p)"), pass through an electron lens system composed of lens electrodes 3 and 4, and reach a target (screen) 5, converging from directions within a certain angle (an angle made between the travelling direction of the outermost electron when received on the target and the line normal to the gate face is, hereinafter, referred to as the "electron beam incident angle (q)").
In devices such as the electron-beam exposure system, the Braun tube, the electron-beam welding device, the electron-beam heating device, the electron-beam machining device and the like, the current density on the target must be particularly high. For this reason, it is essential that the field emission cold cathode is capable to emit electrons of a high current density. Furthermore, the electron lens and the target therein should have good focus characteristics to achieve the high current density.
In a device of this sort, however, when a large current is taken out from the emitter, the electron beam tends to spread with increasing current, because of the interaction between electrons owing to the Coulomb repulsion, and, as a result, some electrons strike lens electrodes 3 and 4 shown in FIG. 1. In this condition, even if the degree of vacuum is heightened, adsorption gas dissociates from the lens electrode struck by electrons, and this gas turns into cations, which fly towards the emitter. Especially in the case that the potential difference between the lens electrode and the emitter is 100 V or more, the cations are accelerated enough to collide with the emitter. Meanwhile, in the field emission cold cathode element, the distance between the gate electrode and the emitter is equal to or less than several micron and besides the potential difference between these two is normally equal to or more than 50 V. Consequently, under the influence of the electric field thereof, cations colliding with the emitter ionize the residual gas further, amplify the ion current in a manner of avalanche, and give rise to the electric discharge. In this state, if no mechanism to limit the current exists on the side of the emitter, the current flows so much as to melt the emitter, resulting in the breakdown of the element. Even when provided with a resistance or a current limiter structure for the purpose of preventing the electric discharge, cations sputter the emitter material, in addition to giving a shock, which deteriorates the emitter and causes a reduction of the electron emission.
Therefore, it is a matter of great importance that the electron beam divergence angle is made small by keeping the Coulomb interaction between emitted electrons under control. Nevertheless, neither the behaviour of electrons in the field emission cold cathode element has been well understood nor the effect of the Coulomb interaction has been quantitatively measured in the experiments.
Moreover, there are demands that the electron beam is made to converge to a still smaller region on the target. Although it is empirically known that the area of this region depends on the size of the emitter and the gate voltage, the relationship between characteristics of the emitter and characteristics of the electron lens in use has not been much understood yet.